1. Field of the Invention
This invention relates to a recording apparatus utilizing print head control apparatus having a plurality of print elements and an IC control circuit for driving the thermal elements and to a method of driving the thermal elements and, more particularly, to a thermal recording apparatus with selection means for driving a plurality thermal print elements in a thermal print head of the thermal recording apparatus.
2. Description of the Related Art
U.S. Pat. No. 4,364,063 discloses a thermal recording apparatus in which a line of resistive type thermal elements are selectively driven according to current and previous (historical) recording signals. The drive of the thermal elements on not only the current but also the previous recording signals is done in order to avoid excessive heating in case the recording signals require one and the same thermal element to be driven in successive print cycles or print lines. Thus, in a current print cycle, less electric energy will be applied to a thermal element that was energized or activated in the preceding recording cycle, than to a thermal element that was not activated in the preceding recording cycle. This is commonly referred to as "historical drive control". To accomplish such a historical drive control, U.S. Pat. No. 4,364,063 employs two or three shift registers and a latch circuit each having a plurality of single bit registers or stages corresponding to the number of thermal elements to be driven. First, the drive data for one print line (one data for each thermal element) are loaded into a first one of the shift registers and then transferred to the latch circuit. In accordance with the data in the latch circuit, a first drive pulse is applied to selected ones of the thermal elements. Then, the drive data are sequentially read out from the first shift register and written into the second shift register while at the same time the previous drive data are read out from the second shift register. The data of corresponding stages of the first and second shift registers are compared with each other and either are the data in the respective register or stage of the first shift register or that of a third shift register modified depending on the comparison result. The modified data from the first or the third shift register are then latched in the latch circuit. In accordance with the data in the latch circuit, a second drive pulse is applied to selected ones of the thermal elements. This process is periodically repeated, each cycle corresponding to one print line. As a result, in a certain cycle, each thermal element to be activated in this cycle, but not activated in the preceding cycle, will be activated during a time corresponding to the sum of the first and second drive pulses, while thermal elements to be activated in this cycle, that have also been activated in the preceding cycle, will be activated during the first drive pulse only.
As will be appreciated from the foregoing description, in this prior art because there is only one latch circuit, it is necessary to completely replace the current drive data and the previous (historical) data each print cycle. This limits the operating speed and increases the complexity of the control system.
For further explanation and understanding of historical drive control, reference is made to U.S. Pat. No. 4,912,485, assigned to the assignee herein and incorporated herein by reference thereto.
Another prior art disclosed in JP-A-208281/1982 seeks to eliminate this problem by providing two latch circuits and connecting the shift register to both latch circuits. In this case, one latch circuit is employed to store past drive data while the other stores current drive data. The drawback of this method is the increased cost incurred by providing an additional latch circuit.
Moreover, the printing speed in thermal recording apparatus is of paramount concern. Take, for example, a journal or receipt thermal printer employed in a point-of-sale (POS) printing system, such as, an electronic cash register, that employs a thermal print head having 512 dots corresponding to 512 thermal elements and capable of handling a 4 cm. to a 6 cm. paper width. Correspondingly, the thermal print head must have a 512 bit stage, shift register. In the case of providing input drive data to a shift register utilizing a transfer rate of 4 Mbits/sec (4 MHz cycle), it will take 0.25.times.512=128 .mu.sec., i.e., for one dot line of a printing period, it will take 128 .mu.sec. for the transfer of the data for one print line. When the thermal recording apparatus also executes a historical control system in accordance with U.S. Pat. No. 4,364,063, it will take 512 .mu.sec. for exchanging the data for historical control and for providing input of new drive data. Furthermore, taking into consideration the necessary time period for activation of the thermal elements, the time interval for the successive print cycles will become greater thereby reducing printing speed. Under these circumstances, the data transfer time becomes sufficiently large as not to be ignored, such as, in the case where an extended print line length in a thermal print head is involved, such as, in the case of a facsimile machine employing A4 size paper width.